list 是否有任何 haskell 函数将列表与分隔符连接起来？
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Is there any haskell function to concatenate list with separator?
提问by Fopa Léon Constantin
Is there a function to concatenate elements of a list with a separator? For example:
是否有使用分隔符连接列表元素的函数？例如：
> foobar " " ["is","there","such","a","function","?"]
["is there such a function ?"]
Thanks for any reply!
感谢您的任何回复！
回答by Niklas B.
Yes, there is:
是的，有：
Prelude> import Data.List
Prelude Data.List> intercalate " " ["is","there","such","a","function","?"]
"is there such a function ?"
intersperseis a bit more general:
intersperse更笼统一点：
Prelude> import Data.List
Prelude Data.List> concat (intersperse " " ["is","there","such","a","function","?"])
"is there such a function ?"
Also, for the specific case where you want to join with a space character, there is unwords:
此外，对于您想要加入空格字符的特定情况，还有unwords：
Prelude> unwords ["is","there","such","a","function","?"]
"is there such a function ?"
unlinesworks similarly, only that the strings are imploded using the newline character and that a newline character is also added to the end. (This makes it useful for serializing text files, which must per POSIX standard end with a trailing newline)
unlines工作原理类似，只是使用换行符对字符串进行内爆，并在末尾添加一个换行符。（这对于序列化文本文件很有用，根据 POSIX 标准，文本文件必须以尾随换行符结尾）
回答by Ilya Kharlamov
It's not hard to write one-liner using foldr
用foldr写单行不难
join sep xs = foldr (\a b-> a ++ if b=="" then b else sep ++ b) "" xs
join " " ["is","there","such","a","function","?"]
回答by Alexis
Some other ideas of implementations of intersperse and intercalate, if someone is interested:
如果有人感兴趣的话，还有一些实现 intersperse 和 intercalate 的其他想法：
myIntersperse :: a -> [a] -> [a]
myIntersperse _ [] = []
myIntersperse e xs = init $ xs >>= (:[e])
myIntercalate :: [a] -> [[a]] -> [a]
myIntercalate e xs = concat $ myIntersperse e xs
xs >>= fis equivalent to concat (map f xs).
xs >>= f相当于concat (map f xs)。
回答by Alaya
joinBy sep cont = drop (length sep) $ concat $ map (\w -> sep ++ w) cont
回答by Zoey Hewll
If you wanted to write your own versions of intercalateand intersperse:
如果您想编写自己的intercalateand版本intersperse：
intercalate :: [a] -> [[a]] -> [a]
intercalate s [] = []
intercalate s [x] = x
intercalate s (x:xs) = x ++ s ++ (intercalate s xs)
intersperse :: a -> [a] -> [a]
intersperse s [] = []
intersperse s [x] = [x]
intersperse s (x:xs) = x : s : (intersperse s xs)
